1. Field of the Invention
The present invention relates to a fuel cell system and an activation method for a fuel cell.
2. Description of the Related Art
A fuel cell directly converts chemical energy, which is obtained by chemically reacting hydrogen with oxygen, into electrical energy.
In view of the fact that energy density of hydrogen itself is high and that it is unnecessary to provide an active material to a cathode side because oxygen is taken from external air, an energy capacity per volume and weight can be remarkably increased as compared with a conventional battery.
Of fuel cells, a polymer electrolyte fuel cell (PEFC) is an all-solid type using a flexible polymer film as an electrolyte membrane, and thus has characteristics such as excellent handling properties, a simple structure, and rapid start and stop in operation at low temperature. Accordingly, the polymer electrolyte fuel cell is suitably mounted to a portable electronic device.
The polymer electrolyte fuel cell fundamentally includes a polymer electrolyte membrane having proton conductivity, and a pair of electrodes provided on both sides of the polymer electrolyte membrane.
The electrodes include: a catalyst layer formed by mixing a conductive carbon particle carrying platinum black or platinum group metal catalyst, in proton conductive polymer electrolyte; and a gas diffusion electrode formed on an outer surface of the catalyst layer, for supplying a gas and collecting current.
In the polymer electrolyte fuel cell, an assembly in which the electrodes and the polymer electrolyte membrane are integrally formed is referred to as a membrane electrode assembly (hereinafter, referred to as “MEA”).
The polymer electrolyte membrane is required to have characteristics such as proton conductivity, a gas barrier property, an electron insulating property, chemical/electrochemical stability, heat resistance, and a high mechanical strength.
To satisfy those requirements, a perfluorosulfonic acid-based ion-exchange resin is suitably and widely used.
In the perfluorosulfonic acid-based polymer electrolyte membrane, it is necessary for water to move along with the conduction of protons. In a case where a water content of the polymer electrolyte membrane is small, the proton conductivity is low, and in a case where the water content is large, the proton conductivity is high. In the case where the proton conductivity of the polymer electrolyte membrane is low, an internal resistance of the fuel cell is significantly increased, whereby fluctuation of the water content of the polymer electrolyte membrane greatly affects power generation. For this reason, it is important to increase the water content of the polymer electrolyte membrane and to devise a method for maintaining the membrane in a humidified state.
Up to now, in order to humidify the polymer electrolyte membrane, the following methods have been employed. That is, there have been employed a method in which a fuel gas is caused to flow in a humidifier (bubbler tank) and to bubble for humidification and then the fuel gas is supplied to a fuel cell, and a method of directly supplying water through a porous plate provided in a fuel cell, a method of supplying a part of a coolant, which is caused to flow in a stack, as humidification water to the MEA, and the like.
However, in the fuel cell for a portable electronic device, it is desirable to omit as many as possible unnecessary auxiliary appliances so as to fabricate the fuel cell with a small size.
In a fuel cell in which a power generation reaction is performed, the proton generated at the fuel electrode moves (electric osmosis) toward an oxidizer electrode side along with water in the polymer electrolyte membrane. At the same time, water is generated at the oxidizer electrode, so that a concentration gradient of the water is generated in the polymer electrolyte membrane.
For this reason, inverse diffusion of the water from the oxidizer electrode toward the fuel electrode is caused, which contributes to the humidification of the electrolyte membrane.
In order to humidify the polymer electrolyte membrane by a power generation reaction and to promptly change the electrical characteristics into a steady state, it is desirable that a current having density as high as possible flow into a fuel cell unit within a range in which the fuel cell unit is not affected.
However, when the current having high density is forced to flow through an external power supply in a state where the water content of the polymer electrolyte membrane is small and the internal resistance is high, a proton supply rate control is caused, which may lead to the deterioration of the fuel cell due to generation of polarity inversion.
Accordingly, it is necessary to perform an activation treatment for stabilizing the electrical characteristics of the fuel cell without causing the polarity inversion.
As a conventional method for performing the activation, Japanese Patent Application Laid-Open Nos. 2000-277136, 2004-047427, and 2005-093282 disclose an activation treatment method of connecting a resistor between a fuel electrode and an oxidizer electrode of a fuel cell prior to supplying power to an electronic device at the time of starting the fuel cell.
In the above-mentioned method, when the resistor is connected between the electrodes, that is, the fuel electrode and the oxidizer electrode, a short-circuit current is caused to flow due to power generation, and produced water generated at the oxidizer electrode at this time is diffused to humidify the polymer electrolyte membrane, thereby activating the fuel cell.
By connecting the resistor between the electrodes, a maximum current corresponding to the activation state of the fuel cell is caused to flow as the short-circuit current. Thereby, the possibility of generating the polarity inversion due to the supply rate control of proton can be reduced compared with an activation method of forcing current to flow.
After the determination of the end of the activation treatment of the fuel cell, generated power is switched from a resistor to the supply for the electronic device, thereby enabling a stable power supply to the electric device.
However, the conventional activation treatment methods for the fuel cell have the following problems.
In the activation treatment methods for the fuel cell or in a starting method using the activation treatment method, the control of the activation treatment is performed while monitoring values of a current, a voltage, an internal resistance, temperature, and the like. Then, a threshold is set for a single parameter or a plurality of parameters, and the set threshold is used as a start point and an end point of the activation.
However, these methods require continuous monitoring for detection of the parameter, which leads to excess power consumption.
Further, in a case where a plurality of units for detecting the parameter are combined, the number of auxiliary appliances is increased, which is inconvenient for reduction in cost and size.
Further, Japanese Patent Application Laid-Open No. 2000-277136 discloses a method of controlling time from the start to the end of activation treatment by using a timer, but does not clearly disclose the time setting. On the one hand, if the set time is short, activation is not sufficiently performed. On the other hand, if the set time is extremely long, there arises a problem of flooding in which a flow path for an oxidizer gas or a fuel gas is blocked by excess produced water.